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If co-moving with a photon

  1. Aug 7, 2011 #1
    Knowing that a photon can not be at rest, I was wondering what would happen supposing if we travel at the same constant speed as light in parallel or in front of a specific ray of light.

    Would that ray of light seems to be still for us?

    Would the momentum mass of the photons of that specific light ray all turn out to be 0 since now it have come to rest?

    Therefor, does that ray of light and it's photon relatively in a sense cease to exist from our point of view due to the synchronized co-moving? From light, it will simply turn out into emptiness?
  2. jcsd
  3. Aug 7, 2011 #2


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    Please start by reading the FAQ subforum in the Relativity forum.

  4. Aug 7, 2011 #3
    After having read the FAQ subforum, I still have hard time conceiving certain ideas which still are not clear to me. Especially about the "Rest frame of a photon"

    While understanding every word of that post, it failed to help me confirm or reject my understanding of some ideas:

    "In the rest frame of any object, the velocity of the object itself is tautologically zero. Relativity says that photons always move at the speed of light, c. Always. So right away we have a conflict; relativity says a photon would have to be moving at c in its own rest frame."

    I understand that asking a photon to move at c in it's own rest frame would be problematic, though what I am asking in my OP is completely different. I wanted to know more precisely what would we be able to observe of the light if we happen to move a the speed of light in front on that same light ray. Would for the observer the light stop to exist in his frame of reference?

    "Time and length cease to have meaning in the limit v→c. In that limit, all time and length intervals shrink to zero. In the rest frame of a photon, the coordinates of any point in the universe at any time in the past, any time in the future is identically zero. That just doesn't make a bit of sense."

    On this one I am not so sure to understand. Does it mean that time and length cease to have meaning by framing light velocity to zero from an outside observer? Reason I am confused is because I don't see how it answer my question about a hypothetically situation where we would be traveling in space at the speed of light, not fixed at one point in space nor frozen in time but towards a specific direction where a light ray behind us or parallel to us would be heading as well.

    Maybe I just have horrible formulated my OP question. In that case I will make one simpler.

    If we travel at the speed of light, what will be observed by us about the lights traveling in the same direction as us during the whole time of the travel.

    Sorry, if all this might seem a stupid question, it is just really not obvious for me, therefor I rather not dwell in misconception. Thank you in advance!
  5. Aug 7, 2011 #4


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    We, as massive bodies, cannot move along with a photon, according to relativity theory. Any attempt to assume otherwise denies the validity of relativity, so you cannot expect to get sensible answers from relativity about any questions related to this. There is no alternative theory known that allows massive bodies to move along with a photon, but nevertheless also agrees with all the other aspects and predictions of relativity that have been verified experimentally.
    Last edited: Aug 7, 2011
  6. Aug 7, 2011 #5


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    What all that says is you can't have an observer moving at the speed of light- if you did, light would have speed 0 relative to him and that is impossible.
  7. Aug 7, 2011 #6


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    The best you can do is ask, what happens in the limit as an observer's velocity approaches c, in the same direction as a photon. In that limit, the photon still always travels at speed c for that observer. However, the photon's frequency (and hence energy) would approach zero.
  8. Aug 7, 2011 #7


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    Think about how we actually observe something. We either shine a light on it and see the reflection of that light coming off it to formulate an image of it or we observe something that is giving off its own light, correct? For a photon, what is it that is giving off its own light? A photon doesn't just keep giving off its own light, does it? And if it did, and you were traveling at the same speed in the same direction, how would the light coming from the photon ever catch up to you so that you could see the photon?

    I ask these questions only to show you that the questions cannot possibly have any meaningful answers. That is what the FAQ is pointing out. You cannot reformulate the question to make it simpler. It's an oxymoron type of question once you think about the implications of it. It's not your fault, you just need to recognize that no formulation of the question will make it any simpler.

    One more thing, you're probably not going to feel good about these responses until you understand Special Relativity which is really not that hard, at least it doesn't seem hard once you understand it. The main problem is getting rid of all the misconceptions about SR so that you can get down to what it's really all about. One of the best ways to learn is to read lots (hundreds) of threads on this forum and see what other people went through in their journey to understanding SR. That's what I did even when I thought I understood SR. You can find hundreds of other people that had your very same question.
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